In general, a contour shape of a tread portion of a heavy duty tire in a tire meridian section under a state applied by no inner pressure is formed by a single circular arc.
If such a tire is mounted on a rim and applied by an inner pressure, the expansion of the tread portion becomes different between the vicinity of the tire equator and the vicinity of a tread ground contact edge, and so called shoulder dropping phenomenon is caused in the contour of the tread portion.
In a tread portion where a shoulder dropping phenomenon occurs, the ground pressure at the tread ground contact edge becomes insufficient, and partial uneven wear called shoulder wear is liable to occur.
Since a tire in which uneven wear has occurred becomes a cause of vibrations of the car, the tire is uninstalled early, and a sufficient tire life can not be obtained.
Therefore, in order to prevent the above-mentioned shoulder wear, a heavy duty tire in which the contour shape of the tread portion is formed by a 1st circular arc having a center on the tire equatorial plane and a 2nd circular arc disposed axial outside the 1st circular arc and having a radius larger than the 1st circular arc, has been disclosed in the following patent document 1 for example.